1. Field of the Invention
The present invention relates to support of delay-sensitive services in a wireless local area network (WLAN), and more particularly, to a method of providing real time service, such as voice over Internet Protocol services, over a WLAN while taking into account such things as WLAN station throughput, power consumption and quality of service.
2. Description of the Related Art
A wireless local area network (WLAN) is one in which a mobile user can connect to a local area network (LAN) through a wireless/radio connection. Standards, such as IEEE 802.11, have been developed to specify the technologies for WLANs which are already commonly used in enterprise environments. However, WLAN is presently used for primarily non-real time data exchanges, such as email, web browsing, and file transfers. Because WLAN was not originally intended to support real time services, there are a number of issues and problems with using it for real-time applications, such as voice over Internet Protocol (VoIP). Specifically, when using WLAN for VoIP, there is a trade-off between terminal power consumption, overall system throughput, user throughput and quality of service requirements.
802.11 standards ensure that all stations, both radio-based network interface cards and access points, implement access methods for sharing the air medium. The basic 802.11 standard therefore mandates that all stations implement a Distributed Coordinated Function (DCF), that is, a form of carrier sense multiple access with collision avoidance (CSMA/CA). CSMA is a contention-based protocol for ensuring that all stations first “sense” the medium before transmitting data. The main goal of using DCF is to avoid having multiple stations transmit data at the same time, which results in collisions and corresponding retransmissions. Hence, DCF for the basic 802.11 standard is a contention-based protocol that does not support quality of service requirements. Because there is no way to guarantee that a station will be able to send its data within a certain time interval, in the DCF mode of operation, VoIP data can be subjected to intolerable days. Additionally, a basic WLAN in DCF modes is found to be quite inefficient when attempting to carry VoIP traffic such that various requirements of VoIP, for example, voice quality and delay, are met. For example, a single 11 Mbs WLAN access point may only serve on order of 10 voice calls, even though the traffic generated by these calls may be much less than 11 Mbs. Thus, the useful capacity of a basic WLAN in DCF modes is, in effect, much lower than one would expect.
To address quality of service issues, the 802.11 standards have evolved and the 802.11e specification provides a new mechanism, Enhanced Distributed Channel Access (EDCA), for resolving contention and supporting quality of service requirements. EDCA improves on the quality of service issues associated with DCF by employing different access traffic classes and access categories, such that it is possible to classify stations as having different transmission priorities. Thus, EDCA dramatically improves the chances that real time application data will be sent on a timelier basis. However, EDCA is still purely contention based, and even with the access categories, it is still possible that requested quality of service levels required for real time applications may not be met.
802.11 specifications do provide for “polled” modes of operation. In the “polled” modes of operation, it is possible for a point coordination function to employ contention free periods in which stations do not contend for transmission, but are instead individually polled to ensure transmission opportunities within a given time period. However, the start time when a station is allowed to transmit can vary. This variability means that the time it takes for all the polled stations to send data can vary greatly, depending on the traffic. This variation also may also mean that real time data, such as voice samples, may not meet real time requirements. In the 802.11e specification, an enhanced polling mechanism, Hybrid Controlled Channel Access (HCCA), may be used. Although HCCA is similar to the “polled” mechanism in basic 802.11 standards that uses point coordination function, it provides additional flexibility in terms of when a station can be polled and how long the station is allowed to transmit data. However, it may be quite difficult to determine when a station can be polled and how long the station is allowed to transmit data because a hybrid controller, which is responsible for polling the stations, has to be aware of information, such as latency, bandwidth and jitter, in order to prioritize how it polls the stations. The hybrid controller resides in the access point and without further knowledge of a station's status, the hybrid controller cannot know for sure that a station even has data to send. In addition, there is a signalling cost associated with polling which causes important overhead when the user data is small, as in the case of voice data, or when the data is nonexistent, as in the case of a silence interval during transmission of voice data.
Currently used request-to-send (RTS) and clear-to-send (CTS) mechanisms allow a station to request that exclusive use of the bandwidth be granted by the access point for a time period. The drawback of these mechanisms, however, is also signalling overhead.
Furthermore, there is a current problem of “hidden” terminal when using the contention based modes of operations. For example, station A might not be able to “hear” transmissions by a “hidden” station B, and hence there can be collisions due to the assumption that the medium is free when station A tries to send data on the medium at the same time as station B. As such, hidden terminals have a negative effect on overall system throughput. The current mechanisms discussed above solve the hidden terminal problem, but the solutions to the hidden terminal problem provided by these mechanisms also generate additional system overhead.